1) Field of the Invention
The present invention relates to a process for producing a polyethylene terephthalate (PET) based polymer from a lower dialkyl ester of a dicarboxylic acid (LDE) and a glycol (GLY) using a specific catalyst system and the addition of small amounts of dicarboxylic acid (DA). This improves not only the ester interchange time but also the polymerization time In particular, the catalyst system comprises manganese (Mn), lithium (Li), cobalt (Co) (optional) and antimony (Sb). More specifically, manganese and lithium are used as catalysts for the ester interchange while cobalt and antimony are used as catalysts for the polycondensation stage. In addition to the catalyst system, the addition of less than about 6.0 weight percent (based on the amount of LDE used) dicarboxylic acid during late stages of vacuum-let-down or after vacuum-let-down further reduces polymerization time and permits lower molar ratios of LDE and glycol to be used.
2) Prior Art
In prior art processes, dimethyl terephthalate (DMT) and ethylene glycol (EG) are typically reacted in the presence of a catalyst (manganese) at atmospheric pressure and at a temperature of from about 180.degree. C. to 230.degree. C. In the presence of a suitable catalyst, these components undergo ester interchange to yield bis (2-hydroxyethyl) terephthalate or "monomer" and methanol The reaction which is conventionally done with 1 mole of DMT and 2 to 2.4 moles of ethylene glycol is reversible and is carried to completion by removing the methanol formed. During the ester interchange, the monomer is the substantial majority product (not considering the methanol) along with small amounts of low molecular weight polymers and unreacted components.
The monomer is then polymerized by a polycondensation reaction, where the temperature is raised to about 280.degree. C. to 310.degree. C. and the pressure is reduced to below 1 mm of mercury vacuum and in the presence of a suitable polymerization catalyst (antimony). From this reaction, polyethylene terephthalate (PET) and ethylene glycol are formed. Because the reaction is reversible, the glycol is removed as it is evolved, thus forcing the reaction toward the formation of the polyester. This known process is described in U.S. Pat. No. 4,501,878 to Adams.
Manganese is the preferred catalyst for ester interchange reactions, but the amount of manganese employed must be strictly controlled. The presence of too little manganese during the ester interchange reaction results in very long reaction times, while the presence of too much manganese results in unwanted side products during the polycondensation reaction, and unacceptable degradation of the polymer resulting in poor color (thus lowering the quality of the polymer). The exact range of manganese which proves to be the most desirable must generally be determined through trial and error because many factors affect the reactivity of the manganese. For example, reaction temperature, reaction pressure, the degree of mixing during reaction, the purity of the raw materials, the presence of other additives, etc., all affect the effectiveness of manganese.
In prior art processes, manganese was employed to obtain suitable ester interchange reaction times, but the manganese must be sequestered after ester interchange or during polycondensation by a polyvalent phosphorous compound to aid in reducing the discoloration and unwanted side products Generally, prior art processes employed about 50 ppm to 150 ppm manganese based on the expected yield of the polymer, as the ester interchange catalyst. Using more than about 150 ppm manganese resulted in polymer degradation even if phosphorous was employed in excess to sequester the manganese. It is believed that this occurred because the phosphorous was incapable of complexing with the manganese to the degree necessary to prevent discoloration.
U.S. Pat. No. 3,709,859 to Hrach et al discloses a multi-component catalyst system for producing polyester. Among the many catalysts mentioned are lithium, cobalt, manganese and antimony. Although these catalysts are set forth in the background portion of the patent, the patent claims a catalyst system comprising antimony, lead, and calcium, and additionally strontium or barium. Hrach et al also teach the necessity of employing pentavalent phosphorous compounds as stabilizers in order to prevent the formation of discolored polyester.
U.S. Pat. No. 3,657,180 to Cohn discloses a process for making polyester resin in which lithium or a divalent metal compound are employed as catalyst The specification states that manganese may be one of the divalent metallic compounds which can be employed. The order of mixing the various raw materials and the addition of the compounds to the process described in the Cohn invention is stated to be critical The process is carried out by reacting DMT and ethylene glycol in the presence of a lithium salt under ester interchange conditions followed by the addition of manganese The process also includes using manganese as a catalyst with lithium being added after the ester interchange reaction. In either case, the second metal is always added after ester interchange, and thus is not used as a catalyst. Moreover, the second metal is always added in a higher than catalytic amount. The second metal is added along with a slurry of glycol and a small amount of terephthalic acid before vacuum-let-down to provide slip for polyester film and the amount added is several factors larger than catalytic amounts.
British Patent No. 1,417,738 to Barkey et al discloses a process for manufacturing polyester in which a preferred ester interchange catalysts may include zinc, manganese, cobalt, and lithium, among others. Preferred polycondensation catalysts include antimony compounds. This reference, however, claims other catalyst compounds and mentions the above catalyst only as background information.
Various patents assigned to Eastman Kodak Company (British Patent Nos. 1,417,738, and 1,522,656; U.S. Pat. Nos. 3,907,754, 3,962,189, and 4,010,145) disclose a broad variety of catalyst systems, including a manganese, cobalt, lithium and titanium combination and a manganese, titanium, cobalt and antimony catalyst system, with phosphorous being used in each of these systems as a sequestering agent. Each of these catalysts was added at the beginning of ester interchange. Although these catalyst systems would generally reduce the overall time required to process the raw materials into polyester, because the ester interchange time was substantially improved; the polycondensation time was not substantially improved.
U.S. Pat. No. 3,487,049 to Busot discloses a catalyst system of manganese, sodium and antimony Furthermore, a small amount of terephthalic acid mixed in a glycol slurry is added to the reactor during vacuum-let-down (at 30 mm mercury) for increasing the polymerization rate, etc.
Improvements which reduce the ester interchange time, but not the polycondensation time, for example, are not particularly advantageous especially where different reactor vessels are employed for the ester interchange reaction and the polycondensation reaction. When different reactor vessels are employed, a reduction in only the ester interchange time, for example, does not necessarily reduce the total process time, because the total process is only as fast at the slowest stage in the process Therefore, a reduction in time for one of the two stages may not improve the overall existing process. In such a situation, additional reactor vessels could be purchased for the slowest stage to improve the total process time, but this is an expensive solution.
In addition to increasing the ester interchange rate and polycondensation rate, it is desirable to use lower molar ratios of GLY to DMT, i.e., lower than 2:1 as is conventionally known. Although chemically GLY and DMT are present in PET in a 1:1 molar ratio, at least a 100 percent excess of GLY (2:1 ratio) must be employed in the conventional process to yield the required high degree of ester interchange and high molecular weight of the polymer, and to prevent side reactions which lower the yield and produce polymer having poor color and clarity Moreover, lower ratios of GLY to DMT conventional processes drastically increase the process time, when a Mn/Sb catalyst system is used.
There remains a need to develop a catalyst system and process which will reduce not only the ester interchange reaction time but also the polycondensation reaction time so that the totality of processing time is substantially reduced.
It is a further aim or aspect of the present invention to not only quickly produce a PET based polyester from raw materials, but produce a polyester which has acceptable clarity, IV and color properties.
It is an additional aspect of the present invention to reduce the GLY/LDE molar ratio below 2:1 during manufacturing of PET based polymer and yet maintain the yield, color, and clarity of the polymer. Furthermore, the lower molar ratio of GLY/LDE with the catalyst system and dicarboxylic acid addition does not significantly lengthen the polycondensation time.